For one such conversion, I cut the hand of the warboss off at the wrist, then built a powerklaw out of sprue bits and two sword blades I stole from some slugga boyz. Ended up looking like he had the klaw grafted on, which looked suibtly Orky to me.

The wonderful thing about orks is that you can get away with a lot of creative license, as long as it looks reasonable. If it's sharp and pointy and looks like it could pinch, it's a powerklaw.